Silicon-based integrated circuits have long been used as a platform for microelectronic applications. More recently, as speed, bandwidth and signal processing requirements have increased, optical systems are now also being integrated on silicon-based integrated circuits.
Thus, instead of or in addition to using silicon to facilitate the flow of electricity, silicon is used to direct the flow of light. While the speed of electricity and the speed of light are the same, light is able to carry more data over a given optical path than can electricity over a given electrical path. Accordingly, there are significant advantages to using light as a data carrier. Furthermore, using silicon as the optical medium allows for application of, and tight integration with, existing silicon integrated circuit technologies. Silicon is transparent to infrared light with wavelengths above about 1.1 micrometers. For telecommunications wavelengths, silicon has a refractive index of about 3.45, whereas silicon dioxide has a refractive index of about 1.44. The tight optical confinement provided by this high index contrast allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers, thus facilitating integration with current semiconductor technologies. In addition, silicon photonic devices can be made using existing semiconductor fabrication techniques used for CMOS circuits, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip.
In practice, silicon photonics can be implemented using silicon-on-insulator (SOI) technology or bulk silicon technology. In either case, in order for the silicon photonic components, such as waveguides, to remain optically independent from the underlying silicon of the wafer on which they are fabricated, it is necessary to have an intervening dielectric material. This is usually a dielectric such as silica (silicon dioxide), which has a much lower refractive, index (about 1.44) than silicon in the wavelength region of interest. Silica is also used on the top and sides of a silicon waveguide core, thus forming a cladding around the entire waveguide core. This results in total internal reflection of light at the silicon core-silica cladding interface and thus transmitted light remains in the silicon waveguide core.
One communications technique which can communicate large amounts of data over a waveguide optical link is known as wave division multiplexing (WDM). A typical example of data propagation in a WDM system is illustrated in FIG. 1. As shown, an optical transmission system 100 includes, for example, a plurality of silicon waveguides 110a . . . 110n, collectively shown as waveguides 110, each carrying data of an optical communications channel. The system 100 includes multiple data input channels 120a . . . 120n, collectively shown as 120, where each data input channel 120a . . . 120n transmits data in the form of pulses of light or as electrical signals. In order to simultaneously transmit the data carried on the multiple data input channels 120a . . . 120n, the data in each data input channel 120a . . . 120n is modulated onto a respective optical carrier having a wavelength λ1 . . . λn by a respective resonant optical modulator 130a . . . 130n. The outputs of the modulators 130a . . . 130n form respective optical communicating channels. The resonant optical modulators 130a . . . 130n are collectively shown as 130. The optical carriers at wavelengths λ1 . . . λn can be supplied to each resonant optical modulator 130a . . . 130n by a highly accurate temperature controlled laser source 136. The modulated light output from each resonant optical modulator 130a . . . 130n is provided to a respective waveguide 110a . . . 110n and the outputs from the waveguides 110 are then multiplexed into a single optical transmission channel waveguide 150 by an optical multiplexer 140. The multiplexed light is then transmitted along the waveguide 150 to an endpoint (not shown) where the data modulated light is de-multiplexed and demodulated before being used by an endpoint device.
The resonant optical modulators 130a . . . 130n, which may be ring modulators, are designed to resonate at their respective carrier wavelengths λ1 . . . λn. The resonant optical modulators 130a . . . 130n have resonant cavities and are constructed of materials with refractive indices, both of which are affected by temperature changes. The changes in temperature of the resonant optical modulators 130a . . . 130n cause their respective resonant frequencies to change and move away from their respective carrier wavelengths λ1 . . . λn. As a result, the modulation index of the modulators 130a . . . 130n drops resulting in reduced signal-to-noise ratio and the potential for data transmission errors. Therefore, there is a need for a WDM optical communications system which can adapt to temperature or other changes which might adversely affect the modulation of data signals onto an optical communication channel by a resonant optical modulator.